drm/i915/gem: Replace dev_priv with i915

Anyone using 'dev_priv' instead of 'i915' in a cleaned-up area
should be fined and required to do community service for a few
days.

Using 'i915' instead of 'dev_priv' has been the preferred
practice over the past years and some effort has been spent to
replace 'dev_priv' with 'i915'. Therefore, 'dev_priv' should
almost never be used (unless it breaks some defines which are
dependent on the naming).

I thought I had cleaned up the 'gem/' directory in the past, but
still, old aficionados of the 'dev_priv' name keep sneaking it
in.

Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tvrtko Ursulin <tursulin@ursulin.net>
Reviewed-by: Tvrtko Ursulin <tursulin@ursulin.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20240328071833.664001-1-andi.shyti@linux.intel.com